A dipmeter is a device equipped with at least three sensor pads mounted on arms extending outwardly from a sonde which is lowered into a well borehole to make measurements. The several sensor pads supported on the sonde include similar detectors which are typically capable of measuring some physical parameter of the adjacent formations. Indeed, four arms are commonly used and six such arms can be used with a highly precise system. For instance, resistivity can be the parameter of interest and it is commonly measured by dipmeters to provide an output indicating dip. As known in the art, a single geological feature or event is located in common to three or more of the dipmeter curves. For instance, in contrasting the resistivity of a rock formation to a salt water saturated porous sand, there is a marked difference in resistance. This creates a common excursion in the signals for all of the sensor pads which detect the rock-sand interface just described and creates in the curve the common feature. The curve provides a signal which physically is located along the path of the particular pad sensor moving along the borehole of the well and provides this signal at the intersection of the borehole with the particular geological feature. Three such sensor pads will determine the dip of the interface of the geological feature; if there are four, this provided redundancy so that the dip angle of the geological feature can be double checked. With six arms, greater redundancy is achieved and greater accuracy in measurement of the geological feature can then be obtained.
The sharpness of the signal is in some fashion dependent on the relative diameter of the sensors on the pad. It also depends on the depth of investigation into the formations adjacent to the borehole. Measurements extend somewhat into the formation, and having a finite diameter at the pad sensor, they effectively measure the geological features giving rise to signal changes at a depth into the adjacent formations; in effect, the measurements are made as though the pad sensors were deployed at a diameter somewhat larger than the physical diameter of the borehole. Typically, borehole diameter is measured with a caliper which is normally mounted on the same tool. Thus, the caliper may measure the borehole diameter to some specific distance, but the diameter of the measured region is somewhat larger than the caliper measured diameter.
This problem can be countered somewhat by simply substituting a large diameter. For instance, if a 7.625 inch drill bit is used and the hole formed thereby is approximately 8 inches in diameter, various models used heretofore have suggested that the measurement diameter or the electrical diameter is the calipered diameter plus about 0.5 inches. This charge in diameter has been supported for sensors on a dipmeter making measurements in an oil based mud. However, in using a sensor pad supporting a focused resistivity sensor the depth of investigation is somewhat larger and suggests a diameter in excess of measured diameter by about two inches. In the example noted, the eight inch borehole will then involve an effective or electrical diameter of ten inches. Even worse, the two inch modification will vary as a function with the contrast in resistivity between the formation dynamically opposite the sensor and the resistivity of the borehole fluids. If two inches is the diameter at the depth of investigation, there will be a very substantial range of errors incorporated in the dipmeter measurements. The present procedure enables a dipmeter of any number of arms to make measurements from the several sensor pads carried on the arms of the device, and assists in controlling the filtering of the dipmeter signals so that the resistivity log from the dipmeter can be modified before determining the formation dip, thereby resulting in improved dip determinations.
The present procedure utilizes a multi-arm dipmeter system having four or six sensor pads in the preferred embodiment wherein resistivity measurements are made; each sensor pad forms its own output and the outputs are provided through a logging cable from the dipmeter tool supported on the logging cable in a well borehole, and these signals are delivered to the surface. Each signal is provided to an adjustable high pass filter. After filtering to reduce or reject certain low frequency components, the signal is then provided to a dip measure computer which makes determinations of dip. This data is then provided to a recorder also having an input from a depth measuring device so that dip events can be located as a function of depth in the well borehole. Moreover, the adjustable high pass filter is provided with a variable frequency set point in accordance with the teachings of the present disclosure. This assists in reducing low frequency content of a specified frequency band, and thereby enables the system to respond to dipping events with greater accuracy. Any possible ambiguity as a result of the depth investigation which implicitly modifies the diameter of the borehole is thereby reduced, and errors in measurement are thereby reduced.